Manchester City is currently experiencing difficulty in winning games, having absolutely everything to bother about this season. Perhaps it's just a temporary condition, which means they will definitely bounce back to winning ways. I'm still surprised to see Manchester City to crumbled this season, I thought they're untouchable but with the current situation on ground, they're just as any other ordinary club in EPL. Injuries affects all clubs in EPL and Manchester City have started suffering from it. Pep Guardiola have missed points on important games this season, they're currently sitting third position on the table.
Player who injured is definitely will affect the result since the substitute player isn't as good as the main line up, I only talk about the skill while chemistry, strategy etc also give a big effect to other players. Not many teams have a balance main line up and substitute player, I think only Brighton have this, whoever who play, the result will not be different.
Actually, Manchester City also has quite good substitute players. But still they will not be the same as the club's core players. So a decline in the club's performance cannot be avoided. Especially if the player who is injured is a key player or an important player who is irreplaceable at the club. And De Bruyne is an important player who also makes the ball flow from midfield to attack line very well. De Bruyne and Haaland already understand each other. So De Bruyne's absence also makes Haaland appear to be having difficulties in attack.